1. Field of the Invention
The present invention relates generally to a diesel engine control on engine-stop, and particularly to a diesel engine-stop control system that serves to reduce or suppress vibrations liable to occur when stopping the diesel engine.
2. Description of the Prior Art
There have been conventionally known three ways to stop the diesel engine: any system interrupting the flow of fuel to the combustion chamber, a system cutting off the supply of air to the combustion chamber, and a system cutting off both the flows of fuel and air.
In the system cutting off the flow of fuel, turning an ignition key from ON to OFF causes a sudden interruption of the flow of fuel to the combustion chamber. This system has an advantage in which the engine is allowed to come to a halt immediately following the cutoff of fuel. Nevertheless, as fresh air still remains flowing into the combustion chamber, the pressure inside the combustion chamber rises as a piston moves up to top dead center. As a result, when the engine lowering rapidly in rpm comes in matching with the resonance point of the engine, the engine develops violent vibrations, thus providing an uncomfortable ride to the occupants.
In systems cutting off the intake air supply to the combustion chamber, since no fresh air is allowed to enter the combustion chamber, no pressure rise inside the combustion chamber comes into action, thus keeping the engine from becoming violent in vibration. However, the fuel remains injected into the combustion chamber even after the engine has ceased from working, rendering inferior the fuel consumption. In addition, this system has other drawbacks in which it takes a prolonged period of time till the engine stops absolutely, compared with the former system interrupting the flow of fuel and in which a partial vacuum developed in the combustion chamber causes much oil loss into the combustion chamber from the crank case via piston rings, thereby making the combustion chamber so rich in fuel that much smoke is liable to take place in the exhaust emissions from the next combustion cycle. This system, moreover, has a fear of involving the engine in a possible risk in which even if the device to cut off the intake air had trouble, either of fuel and air would remain charged into the combustion chamber, resulting in failure of stopping the engine.
Disclosed in Japanese Patent Publication No. 33419/1987 is a system interrupting either fuel and intake air into the combustion chamber in order to avoid a sudden stop of the engine. In accordance with the disclosed prior engine control system, an intake shut-off valve is first closed prior to bringing the engine to a halt to slow the engine rpm down. Then, a delay circuit such as a relay shuts down the fuel supply to the engine, thereby suppressing the vibrations, which might otherwise happen on stopping the engine.
In the conventional system for stopping the engine disclosed just above, the sudden closure manipulation of the intake shutdown valve results in the abrupt reduction of the engine rpm, thereby still developing a vibratory shock in the engine to possibly provide an uncomfortable ride to the drivers.
In accordance with another prior system for stopping the engine disclosed in Japanese Patent Laid-Open No. 41624/1989, operating the switch to bring the engine to a halt results in interrupting the fuel supply to the combustion chamber. Then, an intake cut-off valve installed in an intake manifold allowing air supply to the combustion chamber is closed in response to a detected condition where the engine rpm has approached a resonance range, thereby providing a high resistance against the reciprocating motion of the piston in the combustion chamber to lower rapidly the engine rpm.
Japanese Utility Model Laid-Open No. 47382/1993 discloses a system to stop the diesel engine, in which the delay control is operated not by electrically but by mechanically. This mechanical delay-control system is composed of a vacuum pump, an intake-air cut-off actuator connected to the vacuum pump via a vacuum tube, a fuel cut-off actuator connected in series with the in take-air cut-off actuator via another vacuum tube, and a solenoid common to both the intake-air and fuel cut-off actuators, which is installed in the former vacuum tube. The intake-air cut-off actuator is arranged closer in distance to the vacuum pump than the fuel cut-off actuator while a suction required in the intake-air cut-off actuator for drawing in air is set large compared with the suction in the fuel cut-off actuator, so that the intake air to the combustion chamber is first cut off, followed by the interruption of the flow of fuel.
Moreover, an intake-air throttle system for the diesel engine is disclosed in Japanese Patent Laid-Open No. 35241/1983, in which an intake throttle valve installed in an intake manifold is controlled to open fully under the loaded operation of the engine, partially in idling and close completely when the engine is stopped.
In the system in which the fuel supply is interrupted with the fuel cut-off valve installed in the fuel line, the operation to stop the engine is a switching operation of two stages of both the supply and the cut-off, and therefore apt to cause vibrations and shocks when the engine is stopped. Moreover, as the fuel cut-off valve is closed following the closure of the intake-air cut-off valve, the possible delay of the closing of the fuel valve allows too much fuel for the admitted air to flow into the combustion chamber, thus causing the major drawbacks of worse fuel consumption as well as much smoke in the exhaust gases. There is yet the problem to be solved in how to reduce gradually the engine rpm when the engine is stopped by the cut-off of the flow of fuel into the combustion chamber.